Pulsed Techniques and Sample Environments James Lord ISIS

Pulsed Techniques and Sample
James Lord
Pulsed Techniques and Sample Environments
Most muon experiments vary the temperature and magnetic field
We can also consider applying:
– Electric fields
– Currents through the sample
– Light (ionisation, excitation)
– Pressure
– Gases (reactions/absorption)
– Strain (static, sound waves)
Why pulsed?
– Direct effect of time-varying environment (eg. RF)
– Observe slow formation of final muon states
– Measure recovery time of sample after a pulse (eg. charge carrier
recombination), or non-equilibrium state of the sample
– At ISIS, muons are only in the sample for 0.1% of the time!
– Higher intensity available (eg. lasers or flash lamps)
– Avoid sample heating (eg. light, pulsed currents)
– Avoid other problems with steady state conditions (eg. charge accumulation
due to electric field)
Practical points
The stimulus must be pulsed at the same frequency as the beam: 50Hz at ISIS,
or a sub-multiple such as 10Hz.
Time the pulse to:
– Before the muons allowing the sample to relax for time t
– Coincident or just after the muon arrival, to interact directly with the muon
– After the muons have decayed, as a check on sample heating
Usually measure in “red-green” mode, 2 sets of histograms
– Red: stimulus applied
– Green: control measurement without pulse
In this example a laser pulse is applied at t=2µs
(GaAs, Shimomura et al)
Photons with energy above the band gap generate
electron-hole pairs, which may interact with the
muon, changing its charge state and causing
Below the band gap, the photons may directly ionise
some muonium centres
Current flow in a type II superconductor is
often accompanied by flux line motion. This
“averages” the usual field distribution for a
flux line lattice
Pulsed to allow higher currents without
excessive sample heating
Internal field distribution (Maximum
Entropy) for various currents
(Pb-In sample, Charalambous et al)
Pulsed transverse fields
Muons initially implanted in a small
longitudinal field, then the pulsed
transverse field is turned on rapidly
compared to the precession
This technique removes
the restriction of the muon
pulse width and allows
study of final states
Electric fields
Electrons are produced as the muon stops in the sample. They may be
swept away from the muon by the E-field, reducing muonium formation.
Sample: GaAs, T=50K (Eshchenko et al)
Muonium (open) and diamagnetic
fractions (filled circles) as a
function of electric field
Variation of the diamagnetic fraction with
switching rate due to charge build-up.
(±8kV/cm applied at electrodes)
Muons can only penetrate a limited thickness of material (cell window)
Thick windows are needed on a high pressure sample cell
For “Surface” muons we can build cells up to 50 bar, eg. for gas experiments
– Collision and reaction rates are pressure-dependent
– Stopping range of muons in the gas depends on pressure
Gas inlet
“Decay” muons can penetrate cells up to a few kbar
– Pressure dependence of solids eg. magnetic moments